An electrocardiogram (ECG) measurement is conventionally performed using a 12-lead system, although formats with alternative number of leads are also known. The production of a conventional 12-lead electrocardiogram (ECG) involves the placement of about 10 electrodes or less (one of which is a ground or reference electrode) at selected points on the surface of a subject's body. Each electrode acts in combination with one or more other electrodes to detect voltages produced by depolarization and repolarization of individual heart muscle cells. The detected voltages are combined and processed to produce 12 tracings of time varying voltages. The tracings so produced are as follows:
LeadVoltageLeadVoltageIvL − vRV1v1 − (vR + vL + vF)/3IIvF − vRV2v2 − (vR + vL + vF)/3IIIvF − vLV3v3 − (vR + vL + vF)/3aVRvR − (vL + vF)/2V4v4 − (vR + vL + vF)/3aVLvL − (vR + vF)/2V5v5 − (vR + vL + vF)/3aVFvF − (vL + vR)/2V6v6 − (vR + vL + vF)/3where, in the standard, most widely used system for making short term electrocardiographic recordings of supine subjects, the potentials indicated above, and their associated electrode positions, are:                vL potential of an electrode on the left arm;        vR potential of an electrode on the right arm;        vF potential of an electrode on the left leg;        v1 potential of an electrode on the front chest, right of sternum in the 4th rib interspace;        v2 potential of an electrode on the front chest, left of sternum in the 4th rib interspace;        v4 potential of an electrode at the left mid-clavicular line in the 5th rib interspace;        v3 potential of an electrode midway between the v2 and v4 electrodes;        v6 potential of an electrode at the left mid-axillary line in the 5th rib interspace;        v5 potential of an electrode midway between the v4 and v6 electrodes;        vG (not indicated in table above) is a ground or reference potential with respect to which potentials vL, vR, vF, and v1 through v6 are measured. Typically, though not necessarily, the ground or reference electrode is positioned on the right leg.        
Correct interpretation of an ECG requires a great deal of experience since it involves familiarity with a wide range of patterns in the tracings of the various leads.
Other lead systems have evolved from improvements in instrumentation that have permitted extension of electrocardiography to ambulatory, and even vigorously exercising subjects, and to recordings made over hours, or even days. For example, in stress testing the electrodes are moved from the arms to the trunk, although the same number of electrodes (10) may be used. The tracings I, II, III, aVR, aVL and aVF are altered by this modification.
Although a 12-lead ECG is considered to be a cost effective heart test, it is to be noted that the requirement for a relatively large number of electrodes plays an important role in determining patient convenience/comfort, costs, such as the cost of the electrodes themselves, and the time required to properly position and fix each electrode to a subject's body.
Multiple formats or lead systems reflect the fact that signals representing the respective lead signals typically contain mutually redundant information. It is also known that, should one electrode be missing or malfunctioning, an appropriate combination of signals from the other electrodes and/or the other leads, if available and functional, can be used to generate a synthesized signal to approximate the lead signal derived from the missing or malfunctioning electrode. To apply this technique, at least some portion of a full 12-lead ECG is recorded, during an analysis phase. The recorded signals are then processed to generate a function, preferably a linear function, which may be applied to the lead signals which are available to synthesize a lead signal that approximates the lead signal that is missing or distorted beyond use. During a synthesis phase, this function is then applied to the available ECG lead signals. Using this technique, a missing lead may be synthesized. For example, U.S. Pat. No. 6,643,539 discloses a method of generating a set of synthesized ECG lead signals from a subset of ECG lead signals using synthesis matrices. The method involves creating a synthesis transform matrix relating the signals from an electrode subset to the signals from the conventional 12-leads for different electrode subset configurations of one or more missing electrodes. If ECG signals are taken with one or more missing electrodes, then appropriate synthesis matrix transform is applied to recover the full 12-lead ECG signals.
Efforts have also been made to reduce the number of electrodes needed to collect the ECG signals in order to reduce costs (e.g., of the electrodes), complexity and the time required for electrode positioning. The Dower “EASI” lead system provides a method for sensing and analyzing activity of the human heart that requires use of a reduced number of electrodes to produce accurate simulations of conventional 12-lead electrocardiograms (see, e.g., U.S. Pat. No. 4,850,370). The A and I electrodes (the second and third electrodes) are placed on opposite sides of the anterior midline of the subject at the same level as the first electrode (the E electrode), while electrode S is positioned over the upper end of the sternum (manubrium sterni). However, over the years it has been noted that the signal coming from the A-I electrode pair often contains a higher than desirable level of electrical artifact, probably generated by the nearby pectoral muscles.
To address this problem, U.S. Pat. No. 6,052,615 discloses a modified “EASI” method where the 4 electrode positions consist of electrode E positioned at the front midline, modified electrode A positioned at the left mid-axillary line, electrode S positioned over the upper end of the sternum (manubrium sterni), and modified electrode I positioned at the right mid-axillary line. (Such E, A, S and I electrodes are from time-to-time collectively referred to herein as the “EASI” electrodes.) Specifically, the modified A and I electrodes are attached to the subject's is body on opposed sides of the anterior midline below the level of the E electrode but high enough so they are positioned over the subjects' ribs or intercostal spaces.
Given the greater mobility with the reduced number of electrodes, methods and systems have also been developed for monitoring a subject's medical condition using ECG signals. For example, U.S. Pat. No. 6,217,525 discloses a method and device that evaluates the electrical activity of a patient's heart using a reduced set of leads to automatically detect and report abnormalities associated with acute cardiac ischemia. The reduced set of leads are derived from ECG data obtained from a reduced number of electrodes (i.e., fewer than ten electrodes as used in a conventional 12-lead ECG system) placed on a patient. Cardiac ischemia is detected based on prior classifications of local features and/or global features of the ECG data. Local features include well known local morphological measures such as ST elevation, T wave amplitude, and QRS area from the ECG data, and clinical information on the patient such as age and sex. Global features include projection coefficients calculated from projecting a concatenated vector of heartbeat data onto separate sets of basis vectors that define signal subspaces of ischemic and non-ischemic ECGs. One or more classifiers evaluate the local features and/or global features to determine whether an acute cardiac ischemic condition is detected.
Attempts have also been made to develop medical telemetry systems to improve a patient's comfort, freedom and privacy by decreasing the number and volume of devices directly or indirectly attached to the patient. Telemetry systems normally comprise a transmitter for transmitting electromagnetic signals, e.g., from a measurement, and a receiver for receiving the electromagnetic signals from the transmitter. The ECG electrodes attached to the patient lead to the transmitter, which is usually carried by the patient. The receiver is typically installed in an operator room. Large systems may have a multitude of transmitters and receivers, such as disclosed in U.S. Pat. No. 6,773,396. Each transmitter normally operates with a corresponding receiver on a certain channel, preferably over a pre-defined carrier frequency. Most telemetry systems are single-parameter telemetry systems, whereby a complete telemetry system comprising sensor and transmitter is required for each parameter to be monitored. U.S. Pat. No. 6,740,033 discloses a multi-parameter wireless telemetry system for medical purposes. In another example, U.S. Pat. No. 6,829,501 discloses a system where the electrodes connected to the patient are in communication with a patient monitor console through a telemetry-based transmitter transmitting a radio frequency (“RF”) signal to one or more antennas connected to the patient monitor console through a conventional RF receiver.
United States Patent Publication No. 2005/0017864 discloses a system which also uses wireless communication. However, the system utilizes circular concentric circuit detector electrodes, which are described as Laplacian electrodes. The signals from the electrodes are processed to detect alarm conditions, such as arrhythmia types. In particular, it should be noted that the Laplacian electrodes do not provide a measure of the voltage at the electrode contact point. Rather, the output of the Laplacian electrodes provides a measure of the two-dimensional Laplacian of the potential (which is proportional to the charge density at the center point of the Laplacian electrode). See, e.g., Bin He and Richard J. Cohen, IEEE Transactions on Biomedical Engineering, vol. 39, no. 11, pp. 1179-1191 (November 1992).
As a result, there remains a need for an ECG system that improves patient comfort and freedom of motion by decreasing wired devices attached to the patient to allow for greater freedom while being worn even all the time. There is also a need for a system that monitors the patient's conditions, and provides an alarm in the event of an alarm condition (e.g., ischemia). There is also a need for a system that reduces the power and bandwidth requirements for monitoring a patient. There is also a need for flexible monitoring of a patient to respond to different contexts. Furthermore, there is a need for a systems that includes an error detecting and correcting functionality to detect an error in the measurement, e.g., if an electrode falls off the patient's body, and to select one or more replacement electrodes.